Many industrial manufacturers face the problem of wastewaters containing heavy metals like arsenic, lead, mercury, cadmium, iron, and aluminum that are produced by their manufacturing process. Circuit board manufacturers, metal finishers, automotive, aerospace, and semiconductor manufacturing, electroplated metal parts/washing, textile dyes, and steel are prime contributors. If dissolved in heavy-enough concentrations in the wastewater stream, they become toxic when they are not metabolized by the body, and accumulate instead in the soft tissues. Heavy metal toxicity can result in damaged or reduced mental and central nervous function, learning disabilities, diminished energy levels, cancers, damage to blood composition, lungs, kidneys, liver, and other vital organs, and even death. Other heavy metals of concern found in wastewater streams include antimony, chromium, cobalt, copper, manganese, nickel, uranium, vanadium, and zinc.
It is therefore necessary for manufacturers to treat these industrial wastewater streams to reduce these heavy metals to acceptable levels before they are introduced into water streams and water bodies that are subject to environmental government laws and regulations. As a result of improper treatment prior to discharge, many heavy metals have been found to exist at harmful levels in ground waters which are destined for potable drinking water. Agricultural, meat packing, mining, and hydrofracking industries also face particular risks of waste water contamination.
A “solution” represents a mixture of two or more individual substances that cannot be separated by a mechanical means, such as filtration. For example, a liquid solution occurs when a liquid, solid, or gas solute is dissolved in a liquid solvent. The liquid solution constitutes an aqueous solution if the solvent is water. Wastewater streams very often constitute aqueous solutions containing one or more contaminants.
Chemical Water Treatment Methods
Chemical treatment methods are known in the industry for processing wastewater streams. In one commonly used method, the wastewater is treated with a caustic agent like hydroxide to adjust the pH of the water so that the metals contained therein form insoluble precipitates. A coagulant in the form of an organic ferric chloride or ferrous sulfate is then added to the water to promote settling of the metal hydroxide precipitates. The precipitate particles settle into sedimentation tanks. A filtration medium like silica sand, diatomaceous earth, carbon, or cloth is then used to capture the remaining metal hydroxide found in the water. But, this process requires very large volumes of chemicals, as well as land-filling or treatment of the resulting toxic metal sludges.
Non-Chemical Water Treatment Methods
Non-chemical treatments of wastewater generally employ a mechanism known as “sorption.” Sorption can involve both chemical and physical processes, but the end result is the transfer of a substance from one phase to another. In other words, sorption is the movement of toxins and contaminants from the dissolved, aqueous phase to the surface of a solid media. Three different types of sorption predominate wastewater treatment technology: ion-exchange, absorption, and adsorption.
Ion exchange is a separation process widely used in the food and beverage, hydrometallurgical, metals finishing, chemical and petrochemical, pharmaceutical, sugar and sweetness, ground and potable water, nuclear, softening and industrial water, semiconductors, power, and many other industries. Aqueous and other ion-containing solutions can be purified, separated, and decontaminated by swapping targeted ions contained in the solution with substitute ions typically provided by ion exchange resins or other substrates.
But ion exchange is also a proven technology for removing dissolved metals or other impurities from these wastewater streams. It represents a reversible process in which the ionized metal or other impurity compound or element changes place with another ionized compound or element on the surface of a medium like an ion exchange resin.
Ion exchange can produce high-purity water (including softening, deionizing, water recycling, and removal of heavy metals) from the wastewater. In a familiar example to many readers, an ion exchange-based water softener works by passing hard water naturally containing an abundance of calcium and magnesium cations through a volume of resin beads containing sodium ions on their active sites. During contact, the calcium and magnesium cations will preferentially migrate out of solution to the active sites on the resin, being replaced in solution by the available sodium ions. This process reaches equilibrium with a much lower concentration of calcium and magnesium cations in solution, thereby “softening” the water. The resin can be recharged periodically by washing it with a solution containing a high concentration of sodium ions, such as a sodium chloride solution. The calcium and magnesium cations accumulated on the resin will migrate off it, being replaced by the sodium ions from the salt solution until a new equilibrium state is reached.
Synthetic ion exchange resins are typically used within ion exchange processes. These synthetic resins commonly are formed of small 0.03-2.0 mm beads made from an organic polymer substrate, such as cross-linked styrene and divinylbenzene copolymers. Moreover, these resin beads will feature a highly developed structure of pores on the surface of the resin, which provide the sites for trapping and releasing ions. These resin beads can be converted to cation-exchange resins through sulfonation, or to anion-exchange resins through chloromethylation.
In wastewater treatment, these ion exchange resins remove the heavy metals from the solution, and replace them with less harmful elements like potassium or sodium. But, this process for producing synthetic resins is expensive. The resin beads are also highly susceptible to “fouling.” While soluble organic acids and bases removed by the synthetic ion-exchange resin are shed during regeneration, non-ionic organic materials, oils, greases, and suspended solids also removed from the water tend to remain on the surface of the resin bead. Foulants can form rapidly on the resin, and can significantly hinder performance of the ion-exchange system. Cationic polymers and other high molecular weight cationic organics are particularly troublesome at any concentration. For certain types of resins, even one ppm suspended solids can cause significant fouling of the resin beads over time. Thus, a prefiltration unit in the form of activated carbon or other separation material may need to be positioned upstream of the ion-exchange unit to remove these organic contaminants before the wastewater is passed through the ion exchange resin, further complicating the water treatment process and its costs. The costs associated with this pretreatment can be substantial.
Additionally, resins require regeneration once the ion-exchange sites have been exhausted, for example, as feedwater flows through a bed. During regeneration of a cationic resin, metal cations that were previously adsorbed from the wastewater flow, are replaced on the resin beads by hydrogen ions. A step known as “backwash” is often employed during regeneration, so that any organic contaminant buildup in the resin can be relieved, thereby allowing free flow of the wastewater through the resin beads. But, chemically-regenerated ion-exchange processes known in the art tend to use excessive amounts of regeneration chemicals, which require periodic and even on-going treatment, as well as safe disposal of the chemical waste. These processes can be complex and expensive to operate.
Another “sorption” separation process is absorption. This is a physical or chemical phenomenon or process in which atoms, molecules, or ions enter some bulk phase, whether it be a gas, liquid, or solid material. The gas, liquid or solid material takes in the other substance, like a sponge soaking in water. But absorption is necessarily limited by the physical capacity of the absorbent substrate, and can require frequent purges of the taken-up substance to replenish the absorbent capacity of the substrate.
Yet another sorption process is adsorption. This represents a process in which atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to the surface of a substrate. This constitutes a surface-based separation process, instead of absorption which involves the whole volume of the substrate material. Like ion exchange, certain adsorbates in the adsorption process are selectively transferred from the fluid phase to the surface of insoluble, rigid particles.
Activation of Carbon-Based Media
“Activation” is the process of treating a material that is high in carbon for purposes of increasing surface area and creating porosity. Materials can be activated either with chemical treatment followed by a thermal step, or with heat treatment alone. Most commonly, carbon materials that have been activated then undergo further chemical treatment in order to change the activity of the surface of the carbon-based material.
Activated carbon substrates have been employed in the water filtration industry for this adsorption separation process. Unlike synthetic polymer resins used in ion exchange processes, these activated carbon materials constitute a form of carbon that has been processed to make it extremely porous with a resulting very large surface area for adsorption of impurities via van der Waals forces or London dispersion forces, or chemical reactions. Due to its high degree of microporosity, just one gram of activated carbon substrate can provide a surface area exceeding 500 m2 (about one tenth the size of a football field). Moreover, such activated carbonaceous materials can be produced from a variety of natural organic materials like vegetable matter, soft woods, cornstalks, bagasse, nut hulls and shells, various animal products, lignite, bituminous, or anthracite coals, straw, petroleum pitch, or peat.
Chemical Activation
When some of the energy required for a reaction is provided by a preceding exothermic chemical reaction, there is said to be a “chemical activation.” Carbonaceous material may be chemically activated by impregnating it with an acid, strong base or a salt like phosphoric acid, sulfuric acid, potassium hydroxide, sodium hydroxide, calcium chloride, or zinc chloride, followed by carbonization via pyrolysis at a high 450-900° C. temperature range. For example peat can be impregnated with phosphoric acid or zinc chloride mixed into a paste, and then pyrolyzed at 500-800° C. to activate the peat, followed by washing, drying, and grinding this chemically activated peat into a powder to produce activated carbon having a very open porous structure that is ideal for adsorption of large molecules.
For example, Soviet Published Patent Application No. 1,142,160 filed by Sokolov et al. discloses an active adsorbent product made from aluminum salt sludge. Organo-aluminum sludge produced in the process of coagulation of aluminum salts in water is thickened to create a concentration of 10-17%. The aluminum hydroxide fraction is used to precipitate out the organic compounds during a process that is called coagulation. The aluminum hydroxide and organic compounds are then treated with sulfuric acid, and then the solid phase is heated at 210-270° C. for 2-4 minutes. This process destroys the organic material to convert it into activated carbon, and some portion of organic material is reacted with sulfuric acid to produce sulfonic acid derivatives. The end product is used to remove organic compounds and metal cations (e.g., nickel and cobalt) from waste water. But, not only does Sokolov use a non-natural starting material, but also he relies upon a combination of chemical activation to produce activated carbon, and chemical modification to produce the SO3− groups on the surface of the product necessary for yielding its cation-exchange properties.
Physical Activation
Alternatively, carbonaceous materials such as coconut hulls or bamboo can be physically activated by exposing the material to an oxidizing atmosphere like carbon dioxide, oxygen, or steam at a very high temperature falling within the 650-1200° C. range. These processes for producing activated carbon do not produce a media with a usable ion-exchange capacity. As an example, U.S. Pat. No. 6,316,378 issued to Giebelhausen et al. discloses the manufacture of shaped activated carbon pellets. Polymer resin, acetylene coke, or pearl cellulose are dried at 250-300° C. Then Giebelhausen carbonizes his material at a very high 850-880° C. temperature without steam. Finally, he thermally activates his carbon pellet product at an even higher 910-915° C. temperature in a hot gas-fired kiln. Steam is used by Giebelhausen merely to prevent explosions.
In another example, U.S. Published Application 2003/0041734 filed by Funke et al. shows a method for producing an ultra-low emission (“ULE”) carbon material. The Funke reference explains that conventional activated carbon materials contain too much water and carbon dioxide constituents to effectively adsorb water and carbon dioxide molecules from a gas stream in need of purification. Therefore, Funke subjects activated carbon with no ion exchange capacity to extremely high temperatures and time in a reactor in order to drive off all the H2O and CO2 molecules from the activated carbon. This “preconditioned” ULE carbon material is then further treated to a second activation process under the flow of an ultra-dried reactive purge gas like ammonia to remove any additional moisture from the ULE carbon material. Devoid of H2O and CO2 molecules, this processed carbon material can readily adsorb new H2O and CO2 molecules from the gas stream by simple adsorption without any ion exchange reaction. Furthermore, such treatment conditions are on the order of 500-700° C. for 24 hours to 5 days. Indeed, these are extreme conditions that in no way resemble normal physical activation.
Pyrolysis of Peat
“Pyrolysis” is related to activation in that material high in carbon content is exposed to heat. Activation often involves pyrolysis, but the end result is to produce a product with increased surface area. Pyrolysis constitutes the decomposition of organic material through heating, and it occurs in an oxygen-free environment.
Peat is a substance that can be pyrolyzed, and comparative studies of the pyrolysis kinetics for coal and peat have been performed. See Durusoy et al., “Pyrolysis Kinetics of Blends of Gediz Lignite with Denizli Peat,” Energy Sources, vol. 23, pp. 393-99 (2001). But, no particular temperature ranges for pyrolysis were determined in this study, nor was any ion-exchange medium prepared.
Common Uses of Activated Carbon
Activated carbon filters are popular for home and small-volume water purification systems, because of the adsorbency of the carbon substance. Activated carbons are known to have a heterogeneous pore structure, which is classified as microporous (diameter of pore <2 nm), mesoporous (diameter of pore between 2-50 nm), and macroporous (diameter of pores >100 nm). Activated carbons have a large adsorption capacity, preferably for small molecules, and are used for purification of liquids and gases. Volatile organic chemicals found in the water are removed via adsorption. But, activated carbon filters are generally not successful in removing dissolved metals like antimony, arsenic, barium, beryllium, cadmium, chromium, copper, mercury, nickel, and selenium from the water. Moreover, the purification efficiency of activated carbon filters is directly influenced by the amount of carbon contained in the filter unit, the amount of time that the water-borne contaminant spends in contact with the carbon, and the contaminant particle size. Hence, activated carbon filters must necessarily contain very large carbon volumes treating very low water flow rates, which makes them comparatively unsuitable for processing industrial wastewater streams.
Peat-Based Sorption Media
It would therefore be desirable to produce a sorption medium from a natural, organic material. However, a balance must be struck between the physical integrity of the form of the sorption medium versus the ability of the medium to serve as an ion-exchanger, adsorbent, or absorbent. Partially decomposed organic starting material like peat inherently possesses ion-exchange and adsorbent characteristics. Peat is composed mainly of marshland vegetation, trees, grasses, fungi, as well as other types of residual organic material such as insects and animal remains, and is inhibited from decaying fully by acidic and anaerobic conditions. It is also abundant in many places in the world. For example, 15% of Minnesota is covered by valuable peat resources, comprising 35% of the total peat deposits found in the lower 48 states in the U.S.
Pellets made from peat are known within the industry. For example, U.S. Pat. No. 6,455,149 issued to Hagen et al. discloses a process for producing peat pellets from an admixture of peat moss, pH adjusting agent, wetting agent, and other processing additives. The resulting granules can be easily broadcast spread on the ground, and returned to their original peat moss form upon wetting to act as a fertilizer. No effort is made, however, by Hagen to activate his pellets to prepare the adsorption or absorption or ion exchange characteristics of their surface, nor are they used as an ion exchange medium. U.S. Pat. No. 3,307,934 issued to Palmer, et al. shows another fertilizer product containing peat, and water-soluble inorganic fertilizer salt like diammonium phosphate, sulfate of potash, or urea. This peat product likewise is not activated, nor is it used as an ion-exchange or adsorption medium. Instead, Palmer uses peat merely as a carrier for his fertilizer salt.
It is also known in the wastewater treatment industry to use pellets made from other natural organic materials besides peat as a pollution filtering medium. For instance, U.S. Pat. No. 5,624,576 issued to Lenhart et al. illustrates pellets made from leaf compost, which are then employed to remove pollutants from storm water. U.S. Pat. No. 6,143,692 issued to Sanjay et al. discloses an adsorbent made from cross-linked solubilized humic acid, which can be employed for removing heavy metals from water solutions. U.S. Pat. No. 6,998,038 issued to Howard contains a detailed disclosure of a storm water treatment system for which the filtering media can include peat. U.S. Pat. No. 6,287,496 issued to Lownds shows a process for preparing peat granules using a binder and gentle extrusion. In U.S. Pat. No. 5,578,547 issued to Summers, Jr. et al., a mixer machine and process for producing peat beads for adsorption of metal cations at dilute concentrations (<10 ppm) is disclosed. Peat and a sodium silicate or polysulfone/methylene chloride binder are fed to the mixer to form a pellet, followed by drying. This binder chemical acts like a glue to fuse the peat fibers together in order to create a stronger peat pellet. But, Summers and others fail to disclose or suggest any thermal activation process step, nor to demonstrate the selective removal of cadmium ions from wastewaters in the presence of high concentrations of zinc ions. See also U.S. Pat. No. 5,602,071 issued to Summers, Jr. et al.
Russian Patent No. 2,116,128 issued to Valeriy Ivanovych Ostretsov teaches a process for producing a peat sorbent useful for removing oil spills from solid and water surfaces. The peat material is dried from 60% moisture to 23-25% moisture, and then compressed at 14-15 MPa pressure into briquettes. Next, these peat briquettes are heated at 250-280° C. without the use of additional hydrophobic chemicals and without air. The humic and bitumen fractions within the peat mobilize to the surface of the peat briquettes to produce a natural hydrophobic coating. This hydrophobic coating is necessary for the peat briquettes to be able to soak up oil. Ostretsov also reduces the moisture of his heat-treated peat briquettes all the way down to 2.5-10% wt. moisture. This significant water reduction assists with the hydrophobic coating formation and frees up the pores in the peat material so that they are available to soak up oil. Unfortunately, Ostretsov's aggressive thermal treatment of his peat material will reduce hardness, but he does not need to worry about hardness in his peat briquettes, because he does not force water through the briquettes under pressure during waste water treatment. Instead, he merely floats his peat briquettes on the water surface to soak up the oil spill. Indeed, this is not an ion-exchange medium.
Russian Patent No. 2,173,578 also issued to Ostretsov discloses a similar peat sorbent product useful for soaking up oil spills on water surfaces. His peat under this patent disclosure is milled at low decomposition and dried to 20-48% moisture, and then compressed under pressure at a force below 10 MPa, and then heated under a carbon dioxide blanket without oxygen for 20-90 minutes “at a temperature of 15-30° C. above the exuding temperature of water-insoluble resins of the carrier.” However, it is clear that Ostretsov's process will produce a hydrophobic coating on the surface of his peat material, which is the opposite of the hydrophilic surface that is required for adsorption of metal cations from wastewater streams.
Peat is a substance that can be pyrolyzed, and comparative studies of the pyrolysis kinetics for coal and peat have been performed. See Durusoy et al., “Pyrolysis Kinetics of Blends of Gediz Lignite with Denizli Peat,” Energy Sources, vol. 23, pp. 393-99 (2001). But, no particular temperature ranges for pyrolysis were determined in this study, nor was any ion-exchange medium prepared.
Peat as an Ion-Exchange Media
Various efforts have been made to prepare ion-exchange mediums from peat starting material which is chemically activated and, in some cases, chemically modified before the chemical activation step. For instance, U.S. Pat. No. 4,778,602 issued to Allen, III teaches a multi-functional filtering medium consisting of highly humified peat which is treated with an alkaline solution to hydrolyze the humic and fulvic acid fractions contained therein. Next, the peat product is treated with a quaternary amine solution to precipitate out the humic and fulvic acid fractions from the peat. After filtering the drying the peat cake, nitric acid or sulfuric acid is added to neutralize the amine to chemically modify the peat to increase its cation exchange sites by either adding SO3− groups to the peat surface structure, or to oxidize the organic carbon to improve the cation exchange capacity. Finally, the peat residue may be treated to a semi-coking process step at 200-1000° C. at a 40 psi pressure, thereby allowing carbonization of peat residue. This will actually destroy the carbon fibers. Thus, Allen actually chemically modifies his peat product to increase the cation exchange sites, followed by chemically activating it to increase hydrophobic adsorption properties. The enhanced cation exchange capacity is also aided by destruction of the carbon fibers via the semi-coking (pyrolyzation) step.
U.S. Pat. No. 6,042,743 issued to Clemenson discloses a method for processing peat for use in contaminated water treatment. Clemenson mixes raw peat with heated sulphuric acid to produce sulfonated peat slurry. After cooling and drying the slurry admixture to a 60-70% moisture content, he adds a binder like bentonite clay to coagulate the acidic peat slurry, extrudes pellets, and then bakes the sulfonated peat pellets in an oven at 480-540° C. This baking step drives off the moisture, but it also destroys the carboxylic acid (COOH) groups. His chemical activation of the peat material via the sulfonation step adds sulfonate groups (—SO3−) to the resulting peat granules. In use, Clemenson's peat pellets adsorb metals by attaching the metal cations to the sulfonic groups due to their opposite charged states. Clemenson chemically modifies the surface of peat, but failed to preserve carboxylic groups (COOH) that naturally occur in peat. In addition, his invention is lacking of any evidence of improving selectivity to sorb cadmium cations from wastewater in the presence of zinc cations. See also U.S. Pat. No. 6,429,171 issued to Clemenson.
In yet another example, U.S. Pat. No. 5,314,638 issued to Morine discloses a chemically modified peat product that can be used as an ion-exchange material. This peat material is air dried and milled to a size of one mm or less; hydrolyzed in an aqueous hydrochloric acid solution to remove the soluble components (sulfuric acid and nitric acid may also be used); further treated in an extractor with 2-propanol/toluene solvent to remove the solvent-soluble bitumen; dried to remove the residual solvent; and then immersed in a hot concentrated sulfuric acid bath at 100-200° C. for 1-4 hours. This is a chemically-modified peat product. The hot sulfuric acid bath process step comprises chemical modification in which the sulfuric acid reacts with the peat fibers to add sulfonate anions (SO3−) to its surface. These anions within the ion-exchange resin attract metals via chemical attachment.
Various efforts have been made within the industry to use granulated and dried peat material as a cation exchange media. More particularly, Soviet Published Patent Application No. 806,615 filed by Peter Illarionovich Belkevich et al. produces a water filter product from pellets comprising a paste made from peat and a precipitate of neutralizing etching solution. This paste and the resulting pellets are produced without any physical activation treatment. Moreover, Belkevich uses his neutralizing etching solution like a glue to hold the peat fibers together in a pellet and therefore obtain the desired granule hardness. Furthermore, Belkevich employs his peat pellets as a filter to remove non-ferrous metals like copper and zinc and petrochemical products from waste water. It is unclear that Belkevich's peat pellets are acting as an ion-exchange material.
Challenges Faced by Peat and Other Natural Organic Materials
But, the large body of available research illustrates the underlying shortcomings for natural peat for use as a sorption medium. In its natural form, peat has low mechanical strength, tends to shrink and swell, and does not allow for hydraulic loading. Moreover, peat and other organic starting materials suffer from a number of other problems that compromise their utility as a sorption medium. For example, prior art activation steps like pyrolysis can cause these materials to lose their ion-exchange capacity. Carbonization may cause considerable shrinkage and weight loss of the materials, as well as loss of natural adsorption properties toward metal ions. Organic sources also generally suffer from non-uniform physical properties. Naturally-occurring organic ion exchange media are unstable outside a moderately neutral pH range. Finally, such natural organic ion exchange media tend to be prone to excessive swelling and peptizing, tend to leach naturally occurring heavy metals into the treated wastewater solution, and do not show evidence of improving selectivity to sorb cadmium cations from wastewaters in the presence of zinc cations, compared with other peat material.
While the processes known in the art for the preparation of sorption material sourced from natural solid organic material like activated carbon have been useful for certain limited adsorption applications, for many other applications it will be necessary to increase the hardness of the ion exchange medium, while minimally sacrificing the media's cation-exchange capacity in the process, minimize leaching of naturally-occurring heavy metals into the treated wastewater solution, and increase the selectivity to sorb cadmium ions in the presence of zinc cations in wastewaters. It is therefore necessary to develop a low-cost process for producing ion-exchange and adsorption media sourced from natural organic starting material exhibiting good natural ion-exchange capacity, increased adsorption capabilities towards heavy metals, minimized leaching of naturally-occurring inorganic and organic compounds, increased selectivity to sorb cadmium ions in the presence of zinc ions, and improved strength so that the medium can be utilized in a wider range of end-use applications, including the removal of heavy metals from industrial wastewaters. It would also be useful to be able to prepare such a sorption media using low processing temperatures without the use of chemical activation with its caustic and corrosive chemicals, and chemical modification with its reliance upon the addition of functional groups to the media to enhance its ion-exchange capacity. Likewise, it would be beneficial to avoid the aggressive chemical modification of the peat or other organic starting material substrate before the chemical activation step.
But, even if a peat or other organic material granule could be produced with appropriate characteristics of hardness and sorption capacity, their use in wastewater cleanup is significantly challenged by the fact that wastewater often includes several different types of metal ions. While a particular granule made from peat or other organic material may efficiently sorb metal ions if they are the only type of metal ions in the wastewater, their performance may markedly deteriorate if two or more types of metal ions are present. Instead of removing a large amount of the toxic first type of metal ion from the wastewater, the active sites on the granules will become filled with metal ions of the second type as well, thereby reducing the capacity of the granules for focusing upon removal of the first type of metal ions. Thus, it would be beneficial to produce a process that can chemically treat the granules after any thermal activation step to enable them to selectively sorb the toxic first type of metal ions over the second type of metal ions that are of less concern for removal. The contribution of selective sorption characteristics to such chemically-modified granules will allow them to target specific metal ion types for removal from the wastewaters on an efficient basis.